The conventional modem transmitter consists of an encoder, filter, and modulator, connected in that order. The encoder takes an integral number of m bits at a time and encodes them into symbols. Thus, if the bit rate is R bits per second, the encoder groups the bits into R/m symbols per second, each symbol being transmitted in a time interval T=m/R seconds, called the signalling or symbol interval. For each of the possible 2.sup.m combination of bits that make up a symbol, the encoder generates a distinct signal; the set of all possible 2.sup.m signals is typically referred to as the signal constellation. If the type of modulation desired is pulse amplitude modulation, then each signal is a pulse, of an amplitude determined by the symbol, e.g., amplitudes of -3, -1, 1, and 3 for quaternary transmission (m=2, 2.sup.m =4). This is called one-dimensional modulation since the symbol determines one parameter, in this case amplitude. For quadrature amplitude (or combined phase and amplitude) modulation, a two-dimensional signal constellation is required. The parameters of the signal are often referred to as the "real" and "imaginary" parts of a complex-valued signal. For instance, for 4-phase modulation, the signal points may be located at (1,1), (1, -1), (-1, -1), and (-1, 1). In general, a one-dimensional or amplitude modulation system is a subset of (i.e., may be produced as a special case of) quadrature amplitude modulation.
In a conventional analog transmitter, the digital values of the symbol sequence are converted to analog voltage pulses and then filtered to limit the signal spectrum to the desired band having a maximum frequency less than the carrier frequency. The filter output is then multiplied with the carrier signal to produce the modulated signal.
In two-dimensional modulated systems (for which an exemplary transmitter structure is depicted in FIG. 1 of Forney, Jr. et al., U.S. Pat. No. 3,887,768), the real and imaginary parts of the complex-valued symbol sequence may be separately converted to a pair of analog pulse streams. The two pulse streams are then filtered by identical analog filters. The output of the filter with "real" input is then modulated by a cosine wave at the carrier frequency, while the output of the other filter is modulated by a sine (i.e., 90.degree. out of phase) wave of the same frequency. The "in phase" and "quadrature" modulated signals are superimposed to create the quadrature amplitude modulated (QAM) signal for transmission.
Stuart et al., U.S. Pat. No. 4,123,710, shows how a similar structure may be used to produce a partial response QAM transmitter, by the inclusion of partial response encoders.
Conventional digital implementations are similar in concept. However, the filtering and modulation operations are accomplished on digital sample streams. Finally, the digital sample sequence representing the modulated carrier signal is converted to analog pulses and filtered to generate the analog waveform.
It is well known that frequency-domain filtering causes a spreading in the time domain, whereby the signal associated with each symbol interval spills over into other symbol intervals. In Baker, U.S. Pat. No. 3,128,343, a similar effect to filtering in the frequency domain is accomplished by time domain signal generation techniques. Two carrier waves are generated, one of which has a phase indicative of the current symbol interval, while the other is generated with the phase to be assigned to the succeeding symbol interval. Both carrier waves are amplitude modulated by raised cosine envelopes, such that, when the amplitude of one carrier wave is at its maximum, the amplitude of the other carrier wave is simultaneously at its minimum. Superposition of the two resulting wave forms produces the line signal.
Scott et al. U.S. Pat. No. 3,988,540 shows a transmitter structure for phase modulation, in which a particular bandpass filter characteristic is selected; then, knowing the carrier frequency, symbol interval and baseband symbol alphabet, the response of the selected filter to each possible baseband symbol is found, assuming that the response ceases ringing after a set number of symbol intervals. Samples of each possible response are stored in a read only memory. Thus, by continuously adding the response samples of the current and a set number of additional symbol intervals (during which ringing is assumed to continue), a composite sample stream is obtained, which represents samples of the final modulated and filtered waveform.
In Stuart et al., U.S. Pat. No. 3,825,834, a modem transmitter is described in which single sideband amplitude modulation (SSB) is accomplished by first passing the data encoded baseband symbols through a Hilbert transform filter pair, next analog modulating the resulting phase split signals, respectively, onto quadrature carriers, and finally adding the two modulated signals to produce the line signal.
A pending U.S. patent application, Kameya, Ser. No. 136,919, filed Apr. 3, 1980, discloses use of a digital phase splitting (i.e., Hilbert transform) filter pair in a double sideband modem receiver.